Electromagnetic compatibility EMC refers to the ability of equipment or system to meet the requirements in its electromagnetic environment and not produce unbearable electromagnetic interference to any equipment in its environment. The following are ten common EMC questions and answers. Through the following questions and answers, you can better understand the knowledge of EMC.
1. When diagnosing electromagnetic interference problems on site, it is often necessary to use near-field probe and spectrum analyzer. How to make a simple near-field probe with coaxial cable?
Answer: peel off the outer layer (shielding layer) of the coaxial cable to expose the core wire, wind the core wire into a small ring (1-3 turns) with a diameter of 1-2 cm, and weld it on the outer layer.
2. Measuring the biomagnetic information of human body is a new medical diagnosis method. This biomagnetic measurement must be carried out in the magnetic field shielding room, which must be able to shield the alternating electromagnetic field from static magnetic field to 1GHz. Please put forward the design scheme of this shielding room.
A: first of all, consider the selection of shielding materials. Since the magnetic field with very low frequency should be shielded, materials with high permeability, such as permalloy, should be used. Because permalloy will reduce its permeability after processing, it must be heat treated. Therefore, the shielding room should be made of assembled plates. Each plate is processed in advance according to the design, then heat treated, transported to the site, and installed with great care. The joint of each plate should be overlapped to form a continuous magnetic path. The shielding chamber formed in this way can have a better shielding effect on low-frequency magnetic field, but the gap will produce high-frequency leakage. In order to make up for this deficiency, the outer layer of permalloy shielding room is welded with aluminum plate to form a second layer of shielding, which plays a shielding role in high-frequency electromagnetic field.
3. Why can’t spectrum analyzer observe transient interference such as electrostatic discharge?
A: because the spectrum analyzer is a narrow-band sweep receiver, it only receives energy in a certain frequency range at a certain time. The transient interference such as electrostatic discharge is a kind of pulse interference, and its spectrum range is very wide, but the time is very short. Therefore, when the transient interference occurs, the spectrum analyzer observes only a small part of its total energy, which cannot reflect the actual interference.
4. When designing the shielding case, what factors should we choose the shielding material according to?
A: from the perspective of electromagnetic shielding, we should mainly consider the types of shielded electric field waves. For electric field wave, plane wave or magnetic field wave with high frequency, general metals can meet the requirements. For low-frequency magnetic field wave, materials with high permeability should be used.
5. In the field of EMC, why is it always described in decibels (DB)?
A: because the range of amplitude and frequency to be described is very wide, it is easier to express it in logarithmic coordinates on the graph, and DB is the unit when expressed in logarithm.
6. In addition to the shielding material, what other factors affect the shielding effectiveness of the chassis?
Answer: affected by two factors, one is the conductive discontinuities on the chassis, such as holes, gaps, etc; The other is the wire passing through the shielding box, such as signal cable, power line, etc.
7. How many aspects can EMC design products from?
Answer: Circuit Design (including device selection), software design, circuit board design, shielding structure, signal line / power line filtering, circuit grounding mode design.
8. What should we pay attention to when shielding magnetic field radiation sources?
A: because the wave impedance of the magnetic field wave is very low, the reflection loss is very small, and the purpose of shielding is mainly achieved by absorption loss. Therefore, we should choose shielding materials with high permeability. In addition, in the structural design, the shielding layer should be as far away from the radiation source as possible (to increase the reflection loss), and holes, gaps, etc. should be avoided as close to the radiation source as possible.
9. When designing the shielding structure, there is a principle: try to keep the cables in the chassis away from gaps and holes. Why?
A: there is always a magnetic field near the cable, and the magnetic field is easy to leak from the hole (independent of the frequency of the magnetic field). Therefore, when the cable is close to the gap and hole, magnetic field leakage will occur, reducing the overall shielding effectiveness.
10. Why EMC design for products?
Answer: meet the functional requirements of the product, reduce the debugging time, and make the product meet the requirements of EMC standards for electromagnetic compatibility, so that the product will not produce electromagnetic interference to other equipment in the system.
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